Interconnect structures in ICs (Integrated Circuit) typically include semiconductor structures, such as transistors, capacitors, resistors, and the like, formed on a substrate. One or more conductive layers, typically formed of a metal or metal alloy, separated by layers of a dielectric material are formed over the semiconductor structures to interconnect the semiconductor structures and to provide external contacts to the semiconductor structures. Vias are formed through the dielectric material to provide electrical connections between conductive layers and to the semiconductor structures.
Barrier layers are frequently used within the vias to prevent or reduce undesirable diffusing of the metal conductor (typically copper or a copper alloy, although other metals or conductors may be employed) into the surrounding dielectric layer (e.g., silicon oxide, FSG, BPSG, a low-k dielectric, or the like). Typically, tantalum and/or tantalum nitride is used as a barrier layer for a copper via/contact structure. Other barrier layers could include titanium, titanium nitride, nitrogen-containing materials, silicon-containing materials or the like.
In conventional processes, a via or contact hole is formed in the dielectric layer, which may comprise a single layer or multiple layers of the same or different materials. The bottom of the via is typically an underlying conductive layer or region, such as an underlying conductor (e.g., copper) of a previously formed conductive layer or an underlying source/drain region or gate electrode of a semiconductor device. The sidewalls of the via are typically formed of the dielectric material in which the hole is formed.
A barrier layer is deposited along the sidewalls and bottom of the via or contact hole. The barrier layer is typically deposited by a process such as chemical vapor deposition (CVD), which results in a barrier layer in which the thickness of the barrier layer on the bottom of the via is thicker than the barrier layer on the sidewalls, where the barrier layer is needed. Because the barrier layer is typically not as ideal of a conductor as the copper material, however, the barrier layer along the bottom undesirably increases the resistance of the resulting contact or via. Variation in the bottom thickness results not only in higher contact resistance, but also in more variation from wafer to wafer and lot to lot in the contact resistance, potentially affecting device reliability and yields.
Therefore, there is a need for a barrier layer that prevents or reduces diffusion along the sidewalls as well as reduces the contact resistance between the via and the underlying conductive material.